1. Field of the Invention
The present invention relates to a thyristor apparatus employing light-triggered thyristors. More specifically, the present invention relates to a thyristor apparatus comprising a protecting means capable of protecting the light-triggered thyristors from an overvoltage.
2. Description of the Prior Art
A thyristor apparatus is widely used as an alternating current/direct current converting apparatus in, for example, a direct current power transmission system for transmitting electric power in the form of a direct current, a thyristor starting apparatus for starting a synchronous motor, and the like.
FIG. 1 is a schematic diagram of a conventional thyristor apparatus employing thyristors being triggered with an electrical signal. Six thyristor arms U, V, W, X, Y and Z are connected between alternating current terminals R, S and T and direct current terminals P and N. Each thyristor arm includes a plurality of thyristors 1 connected in series. The thyristors 1 each may be a thyristor to be triggered with an electrical signal. In the case where a thyristor apparatus 8 is used in a direct current power transmission system, usually the same is used in a high voltage higher than 120 KV and therefore as many as 100 of the thyristors 1 are connected in series. The gate of each thyristor 1 is connected to a trigger signal amplifying circuit 2 which comprises a photosensitive device 3 such as a photodiode, phototransistor or the like. An overvoltage preventing circuit 4 is connected between the anode and the gate of each thyristor 1. Each overvoltage preventing circuit 4 comprises a current suppressing resistor 4a and a non-linear resistive element 4b connected in series. On the other hand, a light trigger signal generating circuit 6 is provided on the part of the ground. The light trigger signal generating circuit 6 comprises a plurality of light emitting elements 6a of such as light emitting diodes and a transistor 6b connected in series between a positive direct current control voltage terminal 6c and a direct current control voltage common terminal 6d. The light emitting elements 6a and the photosensitive devices 3 are coupled by optical fibers 5, respectively. The base of the transistor 6b is connected to an electrical trigger signal generating circuit 7. The electrical trigger signal generating circuit 7 may comprise a trigger signal generator employing a voltage controlled oscillator and examples of such generator are shown in U.S. Pat. Nos. 2,467,765; 3,047,789; and 3,197,691 and U.S. No. 382,015. These United States patents and United States patent application are incorporated herein by reference.
Now an operation of the circuit shown in FIG. 1 will be described. First an operation in a normal case will be described. The electrical trigger signal generating circuit 7 provides a trigger signal determined in accordance with various conditions such as the condition of the direct current circuit, the condition of the alternating current circuit and the like. The transistor 6b is responsive to the trigger signal to be turned on, so that light trigger signals are simultaneously provided from the respective light emitting elements 6a. The respective photosensitive devices 3 serve to convert these light trigger signals into electrical trigger signals. The trigger signal amplifying circuit 2 serves to amplify these electrical trigger signals to provide the amplified outputs to the gates of the respective thyristors 1. As a result, the thyristor apparatus 8 serves as an alternating current/direct current converting apparatus for performing mutual conversion between an alternating current power and a direct current power, for example.
Now a description is given of an operation in the case where an overvoltage is supplied to the respective thyristor arms, for example, to the thyristor arm U. Since the thyristors 1 are damaged when the same are supplied with an overvoltage exceeding the rated voltage, the overvoltage preventing circuit 4 is provided to prevent the same. The non-linear resistive element 4b of the overvoltage preventing circuit 4 is adapted such that a current starts flowing at 3800 V in the case where the rated voltage of the thyristor 1 is 4000 V, for example. Accordingly, if and when an overvoltage is applied to the thyristor 1, a current flows through the non-linear resistive element 4b to the gate of the thyristor 1, whereby the thyristor 1 is turned on. Meanwhile, the current suppressing resistor 4a serves to control a current flowing into the gate of the thyristor 1 at that time. If one thyristor 1 is turned on due to the overvoltage, the above described overvoltage is also applied to the other thyristors 1 and as a result these other thyristors 1 are also turned on. Such a phenomenon as described above occurs in succession and all of the thyristors 1 in the thyristor arm U are turned on, whereby the thyristors 1 are protected from the overvoltage. Meanwhile, although the above described method of protecting the thyristors could cause a commutation failure of the thyristor apparatus 8, this does not entail any problem in an operation of the thyristor apparatus 8. More specifically, such commutation failure occurs only for one cycle and the thyristor apparatus 8 performs a normal operation in the next cycle and thus the commutation failure of one cycle can be neglected.
There are two types of cases where an overvoltage is applied to the thyristors 1, as described previously. One is a case where an overvoltage of such as a lighting surge, a switching surge or the like is applied from outside of the thyristor apparatus 8 and the other is a case where a partial commutation is caused in the thyristor arm, in which case some of the thyristors are triggered while the remaining ones are not triggered, with the result that an overvoltage is applied to the thyristors not triggered. Now an occurrence of the phenomenon of partial commutation will be described in the following.
First a concept of a margin time of commutation of a thyristor will be described. Generally, in order to turn off the thyristor in a conduction state to render it in a non-conduction state, a reverse voltage is applied to the thyristor so that a current may be caused to flow temporarily in a reverse direction. However, if and when a forward voltage is applied immediately at a time point when the forward current of the thyristor becomes zero, the thyristor is placed again in a conduction state, without application of a trigger signal, due to carriers remaining in the thyristor. Accordingly, a period of time longer than a turn-off time of the thyristor is required from the time point when the forward current of the thyristor becomes zero until the forward voltage of the thyristor is applied again. A period of time after the current flowing into the thyristor becomes zero until a forward voltage is applied again to the thyristor is referred to as a margin time of commutation .gamma.. In order to turn off the thyristor belonging to a certain thyristor arm in a thyristor apparatus, a circuit need be designed such that the margin time of commutation may be longer than a turn-off time of the thyristor. However, there could be a case where the margin time of commutation .gamma. is shorter than a turn-off time of the thyristor depending on the operation state of the thyristor apparatus. This will be described in the following with reference to FIGS. 2 to 5.
FIG. 2 is a circuit diagram for a conventional inverter. The thyristor apparatus 8 as shown in FIG. 1 can function as either an inverter or a converter, but to make the description simpler, a case where the thyristor apparatus 8 is used as an inverter will be explained here. Direct current terminals P and N of the thyristor apparatus 8 are connected through a direct current reactor 10 across a direct current power supply 9. Alternating current terminals R, S and T are connected to an alternating current power supply 11. Further, the alternating current terminals R, S and T are grounded through a grounding transformer 12. The direct current power from the direct current power supply 9 is converted by the thyristor apparatus 8 into alternating current power and provided for the alternating current power supply 11. The commutations of the thyristor arms U, V, W, X, Y and Z in the thyristor apparatus 8 are done by the voltages of the alternating current power supply 11; namely, the thyristor apparatus 8 operates as a line commutation type inverter.
FIG. 3 is a chart showing electrical signal waveforms at each section shown in FIG. 2. At the top of FIG. 2, current-carrying thyristor arms are indicated. Below that, the working alternating current potential V.sub.AC and trigger signals TS for the thyristors are indicated. And below that, the voltage V.sub.U of the thyristor arm U is indicated. And at the bottom, the direct current voltage V.sub.PN between the direct current terminals P and N is indicated. After a current has passed through the thyristor arm U, the thyristor arm V is triggered, then, in succession to an overlap angle u, an interphase voltage between the phases S and R is applied to the thyristor arm U as a reverse voltage. Consequently, the thyristor arm U is provided with the margin time of commutation .gamma.. The margin time of commutation .gamma. is usually lasts longer than a turn-off time of a thyristor, but it may happen that it becomes shorter than the turn-off time of a thyristor depending upon variations in supply voltages, load currents and so on. More particularly, when the neutral point of the alternating current power supply 11 shown in FIG. 2 is grounded, it is common knowledge that, if one line of the alternating current circuit is grounded, the alternating current interphase voltage will advance by a phase angle of 30.degree. from a normal state. This will be explained with reference to FIG. 4. FIG. 4 is a vector diagram showing the interphase voltage between the phases R and S. If the phase R is grounded, for example, the voltage V.sub.RS ' between the phases R and S at the time of the occurrence of the grounding will advance by a phase angle of 30.degree. from the voltage V.sub.RS between the phases R and S at a normal state. On that occasion, the reverse voltage applied to the thyristor arm U is also advanced by a phase angle of 30.degree., and consequently the margin time of commutation .gamma. becomes shorter. This will be explained with reference to FIG. 5. FIG. 5 is an enlarged detail of the portion A in FIG. 3. The voltage V.sub.U of the thyristor arm U at a normal state is indicated with a solid line in FIG. 5, when the margin time of commutation is .gamma.. The voltage V.sub.U ' of the thyristor arm U at the time when the phase R is grounded is shown with a dotted line in FIG. 5, when the margin time of commutation becomes .gamma.'. As seen from the drawing, the margin time of commutation .gamma.' at the time the grounding occurs becomes shorter than the margin time of commutation .gamma. at a normal state.
In general, each of the thyristors in each thyristor arm is provided with a voltage dividing circuit comprising a capacitor and a resistor for balancing the voltages to be borne by these thyristors. However, in many thyristors, there are always some with longer turn-off times and others with shorter turn-off times due to unequality in their characteristics from product to product. Hence, if the margin time of commutation obtainable from the circuit lies between the longer turn-off times and the shorter turn-off times, the thyristors with shorter turn-off times will, when the margin time of commutation has elapsed and the forward voltage is applied to them, have already restored the capability to bear the voltage, whereas those thyristors with longer turn-off times will not have become similarly restored. As a result, the thyristor with longer turn-off times will fail in the turning off and will not bear the forward voltage, and the situation thus leads to the state where the total voltage applied to the thyristor arm is impressed only on part of thyristors with shorter turn-off times. In such a manner, certain thyristors are subjected to an overvoltage on account of the partial commutation failure. The overvoltage preventing circuit 4 shown in FIG. 1 is for protecting the thyristor 1 from the overvoltage just described.
Recently, light-triggered thyristors to be turned on by light have come to be used for a high voltage thyristor apparatus. This is because, by the use of the light-triggered thyristors, the photosensitive devices and trigger signal amplifying circuits that were necessary for the conventional electrical signal-triggered thyristor have become unnecessary, the number of component parts has been decreased, the reliability of the thyristor apparatus has been improved, and further its anti-noise property has been bettered. FIG. 6 is a circuit diagram for a conventional thyristor apparatus employing light-triggered thyristors. Only its difference from the thyristor apparatus as shown in FIG. 1 will be explained below. Each of thyristor arms U, V, W, X, Y and Z includes light-triggered thyristors 13 which are connected in series. Optical fibers 5 are coupled with the light-triggered thyristors 13, respectively. Each light-triggered thyristor 13 is turned on by the light trigger signal outputted from the light trigger signal generating circuit 6, and performs a predetermined operation. However, since the light-triggered thyristor cannot be turned on by an electrical signal, there was a problematic point that the apparatus was unable to use an overvoltage preventing circuit 4 as shown in FIG. 1 which protects a thyristor from an overvoltage by turning on a thyristor by an electrical signal. Hence, such thyristor apparatus employing light-triggered thyristors that are provided with effective overvoltage protecting means have been wanted.